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In this post we learn how to transmit Internet data through LiFi using a class D amplifier as the transmitter and an ordinary audio amplifier circuit as the receiver.

How Li-Fi Concept Works

If you are wondering how a LiFi concept could be used for transmitting USB data, this article will provide with all the details you required.

We know that a Li-Fi concept is used for transmitting a digital data across a given premise more efficiently than any other means invented so far, especially because the Li-Fi idea allows the user to transmit the data and additionally illuminate the area where it's been installed, so it's like getting two crucial benefits from a single unit.

Remember our age old film projector device? It's probably the oldest known method of using light for transmitting data (picture).

Although we always had other great means of transmitting wireless data such as Wi-Fi technology, RF circuits, etc, using light for this purpose was never imagined simply because lights have been always considered as low-tech units, and thus underestimated, until the day when Mr. Harald Hass discovered this hidden potential of lights (LEDs), and showed the world how LEDs could be actually used for transmitting data in a much efficient way than any other contemporary techniques.

Since LEDs are semiconductors devices these become perfectly compatible for handling digital data without any form of distortions. An LED will replicate and transmit the input content exactly as it was in the original source, and this property make LEDs extremely easy to configure for the intended purpose.

So far we have understood that Li-Fi is a method in which LED is used for transmitting a high frequency content within an enclosed room, which effectively transforms the LED into a wireless transmitter as well as a light producing device.For example Li-Fi concept can be used for transmitting and receiving a music data by using an LED as the light source and also a wireless music transmitter.

However the biggest challenge is to use a Li-Fi circuit for transmitting Internet data using ordinary parts and without involving complex and difficult to get components or MCUs.

A USB connector basically consists of the following wiring details:

1) +5V
2) Ground
3) +D
4) -D

The +5V and ground are the supply out terminals which are normally used for powering the connected external device.

The +D, and -D are the data communication terminals which produces the complex differential signal across each other in a push-pull manner, meaning the +D is referenced to -D, while the -D signal is referenced to the +D terminals. This is what makes transmitting Internet through LED so confusing and complex.

This forced me to think of an alternative and more efficient design, that could actually transmit an USB internet data through LED Li-Fi circuit, without distorting the actual signal, and by employing ordinary components.

After some thinking I came up with the following circuits which hopefully would enable transmitting internet through LED light.

I modified the design into the required Li-Fi transmitter circuit for making it compatible to internet signals, as shown below:

We can see how the differential music input terminals are used for receiving the internet data, while the output is connected to an LED via a bridge rectifier.

Using a bridge rectifier appears to be a smart idea, otherwise it would be simply impossible to transmit the push-pull signals through a LED, since an LED would simply fail to differentiate between these two signals.

By using the bridge we have effectively enabled the LED to recognize both the halves of the USB signal and send it to the receiver without causing any distortions in the original content.

The Receiver Li-Fi Circuit

Now the next challenge for me was to ensure that the rectified pulsating internet data through the LED is correctly decoded back to the original differential form in the receiver section.

This looked difficult however the simulation could be quite easily accomplished by using a dual supply based power amplifier circuit, for example the 100 watt mosfet amplifier already published in this website efficiently fulfilled the intended purpose as shown below:

The BJTs and the mosfets can be any general propose rated to work with 12V/1amp supply. However if you want a powerful decoded output you could very well keep the original values for the devices and enjoy a powerful LiFi decoded inernet output.

UPDATE:

In the discussed concept we used a class D amplifier for the LiFi transmitter, however a class D amplifier essentially involves PWM for processing the input, which could be highly undesirable for an internet data to go through.

We do not want to distort or modify the complex Internet data in any manner, therefore a class D amplifier perhaps cannot be applied for an internet LiFi.

Now this looks perfect, and seems like the Internet data would be transferred to the LED without going through any sort of artificial transformation.

To start with we can go with this 1 watt amplifier circuit as the Li-Fi transmitter and use a 1 watt LED at the output. The idea will confirm whether the proposed Li-Fi transmitter really works or not.

If you have any further doubts regarding this simple yet seemingly working LiFi Internet transmitter circuit you can feel free to express them in the below given comment box.

Adding a Push Pull Stage

In the above diagram everything looks great and it seems the circuit is ready for transmitting the Li-F- data without any issues, however there seems to be a little flaw in the design.

What happens if there's no data at the input? The LED would simply shut down, and that's something totally unacceptable in a Li-Fi concept. Therefore we must somehow make sure that the LED always remains illuminated regardless of the input variations or presence of an input data.

In order to satisfy this condition, we need to introduce a basic LI-FI BJT push pull stage, which was already discussed in our first Li-Fi article.

The following image shows how to do it:

The above design now looks to be a perfect Li-Fi internet transmitter circuit without any flaws.

All the spy circuits presented below are significantly powerful, hard to trace in their hidden positions, and equipped for grasping even the weakest of whispers in the vicinity. Moreover the designs are capable of transmitting the picked information upto radial distances exceeding 2 kms.

The above extraordinary capabilities have forced the legal authorities to enforce stringent laws against the use of these transmitters without permission, so before you make and use one of these make sure you have all the legal formalities completed.

One Transistor Design

You might have already come across a host of these extremely basic one transistor FM transmitter circuits, however these may incorporate certain drawbacks as mentioned below:

No substantial transmitting range.

No enhanced sensitivity range

Use 1.5V for operating which render limited capabilities.

Among the first in the line, which is probably the simplest is shown in the following circuit diagram.

Surprisingly it does not employ a MIC, rather the antenna coil itself performs a dual function of detecting sound vibrations and also transmitting it into the atmosphere.

The design is void of a frequency determining stage and thus does not come under tuned transmitter circuits (we’ll discuss about these later on in the article).

Circuit Description

The following single transistor FM bug circuit may be understood as follows:

When switched ON, the capacitor 22n inhibits the transistor from switching until it gets charged. A soon as this happens the transistor switches ON via the 47k resistor forcing the pulse through the inductor which feeds back a negative pulse to the base of the transistor discharging the 22n capacitor.

This switches OFF the transistor until 22n yet again charges fully. The procedures take place rapidly generating a frequency across the coil which is transmitted as carrier waves through the connected antenna.

In the course if the coil is subjected to an external vibrational pulse, it’s forced to mount the above explained carrier waves in the air and could be received and retrieved over a standard FM radio positioned and tuned at the same frequency nearby.

The circuit may be expected to work at around 90MHz frequency band.

Using Tuned Circuit

The second example below shows another single transistor FM spy circuit that incorporates a tuned circuit or a frequency determining stage in it.

In the original prototype the coil was created by etching a spiral track layout on the PCB itself, however for optimal gain and performance such etched antenna coil must be avoided and the traditional wire wound type of coil must be employed.

Incorporating Q Factor

Below's another circuit you would like to know about. The circuit basically makes use of the “Q factor” of the tank network achieved from the coil and the capacitor for generating a relatively high voltage. This stepped up potential attributes the circuit with a rather longer range of transmission.

For an improved performance make sure the coil and the capacitor are positioned as close as possible. Insert the coil leads as deep down the PCB as possible in order to make it tightly hugging the PCB. C2 value could be tweaked for achieving even better response from the circuit.

Preferably a 10pF could be tried. The coil is made of 5 turns of 1mm thick super enamelled copper wire, with 7mm diameter.

Better Saturation Capability

The next FM transmitter design is a bit different than the above types. Fundamentally the design could be classified as a common emitter type, unlike the others which are rather common base types with their design.

The circuit employs an inductor at its base which adds a better saturation capability to the device which in turn allows the transistor to respond in a much healthier way.

Adjustable Coil Slug

The next design in the list is much superior to its previous counterparts since it uses a slug based variable inductor.

This enables the transmitter to be tuned by adjusting the slug core using a screwdriver. In this configuration we can see the coil being attached to the collector of the transistor which allows a massive 200 meters range to the design, with a current that may be not more than 5mA.

The MIC stage is isolated from the base with the help of a 1u capacitor and the gain of the mic could be well tweaked by adjusting the series 22k resistor.

This circuit could be rated as the best as far range is concerned however it may lack stability which could be improved, we’ll learn how in the following explanation.

Improved Stability

The stability of the above circuit could be improved by tapping the antenna from one top turn of the coil as shown in the following figure.

This actually enhances the response of the circuits due to a couple of reasons. The antenna gets aloof from the collector of the transistor allowing it to function freely without unnecessary loading, and the slipping of the antenna to the top further allows the relevant side of the coil to get a higher stepped up voltage induced across itself and also the coil generating a higher concentration of transmission power on the antenna.

Although this enhancement may not actual increase the range of the device, it makes sure that the circuit does not get rattled when hand held, or when the grip is encircled close over the circuit inside its enclosure.

Transmitting Music

If you want your bug circuit to transmit music instead of spying or eavesdropping, you would probably find the following design interesting.

The proposed FM transmitter will allow combining a stereo input simultaneously from the source so that the info contained inside both the channels get into the air for an optimal reception.

The design configuration is quite identical to the one that’s discussed above so does not need much of an explanation.

Analyzing a Two Transistor Spy Circuit

Adding a transistor stage to the above discussed single transistor FM transmitters could enable the designs with extreme sensitivity.

An electret MiC itself has a built in FET which makes it very efficient and makes it a stand alone vibration amplifier device. Adding another transistor stage with it enhances the sensitivity of the device to overwhelming limits.

As may be witnessed in the following diagram, the involvement of an extra transistor stage adds up to the gain of the MIC making the entire unit highly sensitive such that it now picks even the sound as low as a pin dropping on the floor .

The extra transistor prevents excessive loading of the MIC thereby ensuring better efficiency to the sensitivity.

Five things that that make the circuit extremely good with it reception are:

The use of a fix capacitor in the tank circuit along with a adjustable trimmer.

A low value coupling capacitor with the MIC sufficient to handle the capacitive reactance of the MIC which may be around 4k at 3kHz.

A 1u coupler is included between the oscillator and the audio amplifier in order to make up for the low impedance rendered by the 47k base resistor.

The coil used is wound practically using super enameled copper wire which ensures higher efficiency than PCB etched type of coil.

The entire circuit could be compactly constructed over a small sized PCB for acquiring better stability and a drift free frequency response.

This little transmitter will allow you to communicate, chat, send music transmission on any standard FM Radio tuned within the existing band, across a radial distance of not less than 500 meters or half a kilometers.

Warning: Using this transmitter could be illegal in your country or area, take appropriate permissions before indulging.

How the Circuit Works

The circuit of this 1.5 watt transmitter is fundamentally configured for driving a tuned RF amplifier stage by an oscillator stage.

Referring to the diagram we find that the BC547 is rigged in a oscillator mode which resembles a Pierce oscillator circuit.

The base of the BC547 is biased by the 10k resistor, and the crucial RF coil is connected across the collector/positive of the transistor.

As soon as power is switched ON, this coil is resonated by the 20pF capacitor across the transistor collector and emitter.

The 33pF capacitor makes sure that the capacitance does not exceed the maximum specs of the design.

The above capacitor also determines and fixes the working band frequency of the circuit which is within 80 MHz and 110 MHz.

The varicap diode is included in order to convert the fed input voice or music signal into riding electrical pulses over the carrier frequency created by the above discussed oscillator stage.

This modulated signal is fed to the base of the amplifier stage consisting of the BD139 transistor via a bocking 33nF capacitor.

The BD139 picks up the signals and matches it up with the tuned network across its collector terminals formed by the two inductors and a couple of capacitive trimmers.

These trimmers must be adjusted precisely so that the input modulated signal is optimally amplified by this stage and results in a maximum transmission output.

The output is terminated through another inductor which removes unwanted harmonics and feeds a clean amplified RF modulated signal over the connected antenna.

Antenna Specifications

The antenna should be a Yagi antenna as used for old TV sets.

The circuit must be attached very close to the antenna, preferably directly with the connecting points of the antenna.

The power supply can be fed from an external source, or a battery may be used for the same.

All the "earth" symbols must be joined together and terminated over a large copper base positioned right under the PCB...this need be done if a designed PCB is not used.

With a well designed PCB, the "earth" points must be terminated with the inbound large copper tracks which should cover the entire area of the PCB running beside the connecting tracks all across the board.

How to Set the presets

The two 10k pot may be used for optimizing the signal strength or the volume of the fed signal which is to be transmitted.

The BD139 will require a large heatsink attached with its tab.

All the coils used for this 1.5 watt transmitter circuit are 0.6mm super enamelled copper wire wound over an air core having a diameter of 5mm.

The TV transmitter circuit presented in this post incorporates the European standard FM frequency for audio and Video up-links.

Referring to the circuit below, Q1 is configured as a preamplifier for amplifying the audio input to be modulated.

Circuit Description

Q2 is fundamentally responsible for conducting a couple of important functions: It amplifies the carrier frequency generated by the tank circuit, and also modulates the input over this carrier waves.

The preamplified audio signals from the Q1 stage is fed to the Q2 stage at its base for the intended modulation actions.

As we know all transmitter circuits require a conventional "tank" circuit involving an inductor and few capacitors for generating carrier waves.Here too a tank circuit becomes imperative and is formed by the insertion of C5, L1. This network essentially generates the crucial carrier waves.

The video signal which needs to be superimposed with the audio signal is applied to the emitter of Q2 through the variable resistor R7 for implementing the intended modulation process.

The composite signal (audio/video) after modulation via the Q2 and the tank circuit stages is further applied to the connected antenna A1 for the final transmission into the atmosphere so that it can be received by a particular TV set in the vicinity.

The proposed TV transmitter circuit requires a well regulated stabilized 12V supply for operating.
Preferably a 12v battery would give better results due to a much cleaner DC free from all possible ripples and noises.

Circuit Schematics

Some Important Points:

Things you need to consider while building this TV transmitter circuit at home:

Preferably use a well designed glass epoxy PCB for this project.

For the inductor L1 use a super enameled copper wire of 24SWG and wind 4 turns with 6mm diameter over any non conducting former such as paper or plastic.

T1 may be replaced with any standard audio transformer the type which were commonly used in olden transistor sets and radios at the output amplifier stage.

The antenna is not much critical, can be any good conductor of electricity, around a foot long, such as a copper wire. You may try different lengths until you get the optimal response from the TV transmitter circuit.

The operating frequency of this unit could be within 50 and 210MHz.The compatibility of this circuit is well with PAL B/C systems.

You may have some fun with C8 which may be tweaked a bit for acquiring extreme accuracy with the performance of the circuit.

The proposed long range transmitter circuit really is very steady, harmonic free design which you can use with standard fm frequencies between 88 and 108 MHz.

Technical Specifications of the Transmitter

This will likely encompass 5km spectrum (long range). It includes an extremely consistent oscillator for the reason that you employ LM7809 stabilizer that is a 9V stabilized power source for T1 transistor and for frequency realignment that may be reached by means of the 10K linear potentiometer.

The output strength of this long range rf transmitter is approximately 1W however may be more significant should you use transistors like KT920A, BLY8, 2SC1970, 2SC1971…

Transistor T1 is employed as an oscillator stage to present a small power steady frequency. To fine-tune the freq. apply the 10k linear potentiometer this way: should you moderate, in the direction of ground, the freq. would probably decrease but when you fine-tune it in direction of + it would climb.

Essentially the potentiometer is needed just as a flexible power source for the a pair of BB139 varicap diodes.

Both of these diodes function as a changeable capacitor whilst you regulate the pot. By tweaking the diode capacitance the L1 + diodes circuit renders a resonance circuit for T1.

Feel free to employ transistors similar to BF199, BF214 however be careful not to use BCs. At this point you don’t receive yet the long range fm wireless transmitter due to the fact that the electric power is fairly reduced, a maximum of 0.5 mW.

How the Circuit Works

Transistors T2 and T3 functions as a buffer stage, T2 as a voltage amplifier and T3 as a current amp.

This buffer stage is vital for freq stabilization simply because is a tampon circuit between the oscillator and the preamp and final amplifier. It happens to be renowned that bad transmitter layouts normally change freq. whenever you alter the finalized stage.

Using this T2, T3 stage this won’t occur again!

T4 is a preamplifier stage and is employed as a voltage power rf amplifier which enables it to produce adequate power to the ending T5 transistor stage.

As is demonstrated T4 carries a capacitor trimmer in its collector, this is definitely accustomed to render a resonance circuit designed to drive T4 to promote more advantageous situations and do away with those undesirable harmonics.

L2 and L3 coils has to be at 90 degrees perspective one to another, this is to prevent frequency and parasite coupling.

The concluding stage of the long range rf transmitter is equipped with any rf power transistor containing no less than one watt production power.

Utilize transistors like 2N3866, 2N3553, KT920A, 2N3375, 2SC1970 or 2SC1971 should you wish to produce a professional fm transmitter with ample power to take care of an extended spectrum zone. Should you use 2N2219 you will definitely get a maximum of 400mW.

Make use of an effective heatsink for the T5 transistor because it becomes slightly warm. Make use of a reliable 12V/1Amp balanced supply of power.

How to Set-up the Transmitter

Begin by building the oscillator stage, solder a tiny wire to T1 10pF capacitor out and hearing a fm radio, tweak the 10k pot until it is possible to “hear” a blank disturbances or maybe if you connect an music base you could listen to the melodies.

With a 70cm cord it is possible to take care of a 2 – 3 meter region simply with the oscillator stage.

Next carry on and construct the remaining of the rf transmitter, utilize correct shielding as suggested in the above explanation.

As soon as you have completed the transmitter design, hook up the antenna or more effectively a 50 or 75 Ω resistive load and make use of this as a rf probe, feel free to use 1N4148 diode in place of the probe diode.

Fine-tune yet again the 10k pot to favored freq. thereafter go to T4 stage and scale down the initial collector trimmer for highest voltage signal on the multimeter.

After that carry on with the subsequent trimmer and so forth. After that get back on the very first trimmer and readjust yet again until you receive the maximum voltage on the multimeter.

For one watt rf power you could possibly ascertain a twelve to sixteen Voltage. The method is P (in watt) is equivalent to U2 / Z, wherein Z is 150 for 75Ω resistor or 100 for 50Ω resistor, nevertheless one should keep in mind that the proper rf power is lesser.

After those modification, in case things are heading nicely hook up the antenna, keep on employing the rf probe, readjust once more all of the the trimmers right from T3.

Guarantee you don’t have harmonics, verify the TV and radio set to determine if there exists fluctuations on the band. Verify this in an alternative area, a long way away from the fm transmitter or antenna.

The unit is all set up to be used for exchanging music, talks, chats across the suggested range and bands.

Firstly about the shielding, what is the most simplest solution to avoid those "parasite frequencies"?

Secondly, what means those 1nF capacitors at the top? Can they be simple in parallel connection or they need to be separated to every transistor like in scheme?

Thirdly, I sent you a photo of transmitter, I didn't turn on amplifier part because my heatsink is coming. Where can I put antenna for testing without amplifier (T5 stage)?

And lastly, how can I modulate those trimmers if I dont have plastic screwdrivers?

Thank you very very much, this is great project.

Your fan, Himzo.

Solving the Circuit Problem

Hello Himzo,

the simplest and the only way to shield the various sensitive stages is by using metal walls between the stages...

the 1nF capacitors should be positioned exactly where these are indicated in the diagram.... the picture which you have shown will never work... transmitter circuits require extreme care as far as their construction and positioning of the components are concerned.

You can never build a long range transmitter successfully on a breadboard, you will have to do it on a well designed PCB which should have a grounded track base layout encompassing all the thinner tracks, only then you can expect the transmitter to work...that too after careful optimization of the trimmers and by employing a compatible antenna.

Stereo FM Transmitter Circuit Diagram

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Swagatam is an ardent electronic researcher, inventor, schematic/PCB designer, manufacturer, and an avid publisher. He is the founder of https://www.homemade-circuits.com/where visitors get the opportunity to read many of his innovative electronic circuit ideas, and also solve crucial circuit related problems through comment discussion.